Maj Gen Robert Mood, the head of the team in Syria, said locals had warned monitors that they would be targeted. The severity of the attack has not been ascertained but no one was hurt, said the BBC which said the UN has 297 unarmed observers in Syria to verify the implementation of special envoy Kofi Annan's six-point peace plan.

The attack took place during a UN security council special session to review the deepening crisis in Syria.

Shelling and shooting are continuing in Homs province and a live video of the shooting was being broadcasted through webcam by an activist. The video cannot be independently verified.

UN chief Ban Ki-Moon voiced his frustration with the lack of finding a solution in a stronglyworded statement.

"How many more times have we to condemn them and how many more ways must we say that we are outraged?" he said. "The Syrian people are bleeding. They are angry. They want peace and dignity. Above all, they all want action."

He said the UN would explore a number of options to tackle the crisis as Syria lurches towards civil war.

Annan said: "Given the level of violence and the actors on the ground, you could say we are drifting, if we are not already in a sort of a civil war. All efforts are being made to ensure that if it were to become a full-blown civil war, it doesn't spread to the neighbours."

Both Ban and Annan conceded that the peace plan has not been implemented fully and once again urged that the truce be upheld.

"The Annan plan remains at the centre of our efforts. We continue to support it. At the same time, in view of the deteriorating situation I would welcome further international discussions on the way forward," said Ban.